1. Field of the Invention
This invention relates to the field of structures for at least partly blocking or enclosing equipment against the passage of electromagnetic interference. In particular the invention concerns a shielding ground plane structure comprising a laminate of conductive material and elastomeric foam, wherein the conductive material is electrically coupleable to a circuit board or the like by means of a spring pin that resembles a twist drill, and pierces the foam and the conductive material to make a connection such as a grounding connection to the conductive material at the spring pin.
2. Prior Art
Shielding to damp or block the passage of electromagnetic interference is well known in connection with electrical equipment which either generates an electromagnetic field or is sensitive to induction of currents from incident electromagnetic fields. Radio equipment, computing equipment and the like often include a grounded conductive enclosure at least for the high frequency portions of the circuitry to block propagation of the fields generated therein and thus avoid affecting nearby circuitry. Such an enclosure may or may not wholly enclose the circuits which generate or are sensitive to electromagnetic interference. In connection with a particular circuit board, for example, a simple grounded conductive sheet disposed adjacent the conductors of the circuit board can reduce electromagnetic interference that otherwise would couple high frequency signals among the components on the circuit board. Electromagnetic radiation incident on the grounded sheet induces currents in the sheet that are dissipated as eddy currents A wide variety of uses of conductive sheets for shielding and the like are possible.
In connection with a sheet which is to function as a ground plane, i.e., a conductive plane which is electrically connected to circuit ground, it would be possible to connect the conductive sheet to ground at various points However, it is useful to consider carefully the particular connections to be made to a ground plane because if even a small electrical potential exists between the points of connection to the ground plane, substantial currents can be generated between the points due to the normally low electrical resistance of the conductive sheet Such currents generate electromagnetic fields which propagate to cause emission of electromagnetic interference and/or high frequency coupling of elements on the circuit card. This problem can be minimized by grounding a number of circuit grounds at a common point forming the reference ground.
Some examples of shielding techniques involving conductive enclosures and/or ground planes are disclosed, for example, in U.S. Pat. Nos. 4,840,571--Minata et al; 3,731,202--Meyers; and 1,641,395--Moore. A full or partial conductive enclosure is connected to the circuit ground of an electrical apparatus to block propagation of electromagnetic interference. Arrangements involving the insertion of conductor pins into mating receptacles for achieving connection of circuit cards to a housing, backplane or groundplane are disclosed, for example, in U.S. Pat. Nos. 4,811,168--Chesnut et al; 4,649,461--Matsuta; 4,510,553--Faultersack; and 3,375,576--Klein et al. Rigid soldered connections between points on multilayer circuit cards are disclosed in U.S. Pat. Nos. 4,933,228--Katagiri et al and 4,933,208--Dorinski.
A conductive sheet or enclosure for blocking electromagnetic propagation may be rigid or flexible. U.S. Pat. No. 1,942,575--Shapiro discloses a material which comprises woven conductive strands forming a conductive fabric. Provided any gaps in the conductive sheet between filaments of the weave are small compared to the wave length of the frequency of interest, such a material will effectively block propagation Flexibility also has advantages in that the material will assume a desired shape. Another form of flexible conductive material is the metal plated plastic fabric disclosed in U.S. Pat. No. 4,857,668--Buonanno, wherein a nylon knit or weave is electroplated to provide a conductive element in a seal or gasket.
It is sometimes desirable, particularly in printed circuit boards, to build a conductive sheet directly into the structure of the printed circuit board. A metal layer which can be photo-etched is applied to one or both sides of a circuit card, and after etching forms the conductors which couple particular circuit pins. The metal on an area of the opposite side can be left as a groundplane, connected to ground on the conductor (component) side to reduce propagation to or from components on the printed circuit board. In U.S. Pat. Nos. 4,933,045 DiStefano et al; 4,654,755 Henderson et al; and 2,955,974--Allen et al, laminates of metal and plastic are disclosed. U.S. Pat. No. 3,222,442--Parker et al discloses using plastic (in particular a polystyrene foam) as a means to position components while an insulating base or circuit card is formed to engage the leads of the components.
The structure needed functionally to effect a groundplane or similar shield is relatively simple, including a conductive sheet and a conductor passing from a circuit ground on the shielded apparatus to a point on the conductive sheet. However, the specific manner in which the sheet and the connection are embodied are subject to practical requirements. For example, the designer must avoid allowing the conductors of the circuit (apart from the grounding conductor) to short against the groundplane The groundplane should be protected from contact with any conductive outer housing to avoid altering the characteristics of the groundplane arrangement In an apparatus subject to vibration, some form of flexible or resilient coupling is needed, and preferably the connection structure is dimensionally tolerant.
According to the present invention, a conductive groundplane is embodied in a laminate of plastic foam. The conductive sheet is preferably defined by a conductive fabric such as a weave or knit of metallic or metallic-plated fibers The laminate includes a relatively stiffer foam layer to define a shape, and a relatively soft foam laminate which insulates between the conductive layer and the circuit being shielded. To effect at least one electrical connection between the conductive sheet and the circuit, a spring pin is mounted on the circuit to protrude toward the groundplane. The spring pin pierces the soft foam layer and engages the conductive layer. Preferably the spring pin includes a post which is soldered rigidly to the circuit, and a movable shaft that protrudes axially from the post at the distal end. The movable shaft can have an enlarged end and a shaft that is helically fluted in the manner of a twist drill The flutes of the shaft can engage a guide on the post so as to twist the shaft with axial displacement relative to the post. Accordingly, when the spring pin is forced through the soft foam into the conductive fabric, the pin pierces through the conductive fabric such that the enlarged end passes through and locks behind the fabric. Any subsequent vibration is countered in several ways. The resilient foam layers hold the pin in position while admitting some relative motion The pin locked in the fabric maintains good electrical connection due to the locking and also due to the fact that the flutes tend to engage in the metallic material of the fabric.
The groundplane arrangement according to the invention can form a ready packing element to be disposed between a printed circuit card or the like and an outer housing. The groundplane material thus forms a resilient vibration resistant lining for the outer housing, providing a very good connection to the conductive sheet, and therefore maintaining effective shielding. Insofar as the outer housing includes conductors, the invention insulates the circuit card from shorting against the outer housing. These advantages are made possible even though installation is extremely simple. The laminate material is simply placed and the circuit card (with protruding spring pin) is urged against the laminate until the spring pin punches through the soft foam to engage with the conductor.
Unlike an arrangement wherein a pin piercing a metal foil tends gradually to enlarge the hole in the foil due to relative motion between the pin and the foil in the plane of the foil, eventually losing electrical contact, the enlarged pin end, helical fluting and engagement with a conductive fabric according to the invention are such that vibration tends to improve the electrical connection of the pin and the conductive fabric.